Spider silk fibers are fibers having high strength and toughness, and are known to have higher strength and toughness than high-tensile steels, nylon 6 fibers, aramid fibers, carbon fibers, etc. In addition, they have an advantage in that oil is not used as a raw material and biomass can be used instead. Some artificial spider silk fibers also have been proposed. For example, Patent Document 1 has proposed a fiber produced by extruding a synthetic protein spinning solution into a coagulation bath with 90% methanol from a spinneret at a speed of 5 to 10 μl/min. Patent Document 2 has proposed a fiber 1 μm or more in diameter and 5 mm or more in length having a tensile strength of 200 MPa or higher. Non-Patent Document 1 has disclosed a drawn yarn having a strength of 1.91 to 2.26 g/d and an elongation of 43.4 to 59.6% obtained by drawing a yarn at a draw ratio of 5 times in a methanol bath and a water bath. Non-Patent Document 2 has disclosed a drawn yarn having a stress of 600 MPa and an elongation of 25% obtained by drawing a yarn at a draw ratio of 5 times. Non-Patent Document 3 has disclosed a drawn yarn having a stress of 280 to 350 MPa and an elongation of 30 to 40% obtained by keeping a yarn in steam for 5 minutes so as to draw the yarn to 5 times.
However, the toughness of conventional artificial polypeptide fibers is not sufficiently high, and hence tenacious fibers having still higher stress have been desired.